Pressure transducers are widely used in the semiconductor industry for ultra-high-purity gas pressure measurement in process lines. A pressure transducer develops an electrical signal that is proportional to the pressure applied. An ideal pressure transducer design minimizes the effects of temperature, mechanical stress, and aging on the output signal so as not to compromise the semiconductor fabrication process. In actuality, transducers respond to environmental effects, and only some of the effects can be easily compensated through signal conditioning. Others are impossible to correct and add unpredictable variation to pressure measurement.
Correctable errors are typically proportional to temperature or pressure. Transducers can have linear and nonlinear (typically second-order) temperature effects for both zero-pressure output and span. The raw span and offset signals with respect to pressure may also require correction and can be nonlinear. These repeatable errors can be readily corrected by the transducer manufacturing using a variety of analog or digital compensation schemes.
In classic analog compensation, linear errors are routinely compensated with passive or active direct current circuits. The second-order errors, while more difficult, can be handled by nonlinear analog feedback via temperature sensors or by digital correction. As long as errors are repeatable, they are correctable and can be eliminated with proper signal conditioning.
While correctable errors are somewhat problematic for the transducer manufacturer, it is the non correctable errors that cause concern for the transducer user. Environmental effects such as humidity, that can affect the accuracy of a transducer can be unrepeatable, and are much more difficult or impossible for the transducer manufacturer to predict and correct. Examples of non correctable effects include case tress susceptibility, hysteresis, creep, and long-term stability.
An example of case stress would be a shift in the offset of the device due to mounting installation-related stress. The degree of stress applied is different in each application and is hard to predict. If the raw sensor element and package are not designed to reject such stresses, it will be impossible for the manufacturer to eliminate the offset instability caused when the user mounts the sensor in the field. Creep, hysteresis, and long-term offset shift are three other effects that can be time, temperature, humidity and/or pressure related and impossible to eliminate. If a transducer exhibits any of these characteristics, the transducer will require frequent calibration that always adds process variation, unpredictability, and replacement cost, and possibly even downtime.
High-purity transducer applications involve stringent material control and manufacturing techniques to ensure fluid purity of semi-corrosive liquids and gasses. Gas delivery lines are often welded into place, which makes component replacement difficult and costly, and also induces welding stress that can cause transducer errors. In addition, high-purity chemical processing, whether for semiconductor fabrication or any other application, inherently involves highly corrosive environments and requires an absence of chemical and particulate contamination. Any surface exposed to process gasses and liquids must be resistant to corrosion, and maintain smooth, non-particle-retaining surfaces. A transducer used in the system must also adhere to these requirements.
The high-purity market has traditionally used capacitive and Wheatstone bridge-based pressure transducers. Each technology has advantages and disadvantages, and selection has always required certain compromises. Wheatstone bridge designs use strain-sensitive resistors strategically placed on a diaphragm or bean, such that the resistance increases proportionally to the strain change. The resistors (typically four) are arranged into a fully active bridge with two increasing and two decreasing in resistance.
Microelectromechanical systems (MEMS) based transducers generally rely on piezoresistive strain gauges to translate pressure into electrical signals. The piezoresistive effect is based on the change in the mobility of charge carriers in a resistor due to a change in mechanical stress, thus changing the resistance. Piezoresistive bridges typically produce over sixty times more signal that foil gauges for the same applied pressure.
What is still desired is a new and improved ultra-high-purity gas pressure sensor for use with low pressures. Among other features and benefits, the new and improved ultra-high-purity sensor will preferably provide greater sensitivity to pressure changes.
Other advantages and features will appear hereinafter.